Rechargeable batteries, especially lithium ion batteries, need to precharge (recovery-charge) from deeply discharged status to avoid stressing the depleted batteries. When a rechargeable battery is deeply discharged and its cell voltage lower than an under voltage threshold VUV, it cannot be directly charged using large charging current. Instead, a pre-charge mode is needed. In pre-charged mode, a small charging current is used, until the battery voltage is charged larger than the voltage VUV, then it can be charged in normal mode, i.e. charging by larger charging current. So, pre-charging mode also called trickle charge or wake-up charge. For lithium ion battery, the threshold voltage VUV is approximately 2.4V˜3.0V for one cell, depending on battery type and manufacturer. The pre-charging current is about 10 mA˜100 mA. However, the normal charge current can be a few hundred milli-Amperes to 1 Ampere depending on the battery capacity.
FIG. 1A depicts the charging profile 50 for a lithium ion rechargeable battery. When the battery voltage is higher than VUV, the battery enters into constant current (CC) charging mode, and a large constant current is used to charge the battery (the battery voltage also increases as the battery capacity increases). When the battery voltage increases to VOV, which represents overvoltage threshold (normally around 4.2V for a LiIon battery), the battery enters into constant voltage (CV) charging mode. In this mode, the charger holds the voltage at VOV. When the charging current decreases to a predetermined minimum value, for example 50 mA, the charge procedure is stopped. During the CV charge mode, the charger must regulate the voltage precisely to VOV (to within +/−0.005 V), otherwise the charging current will not taper off with increasing battery capacity. If, for example, the charging output is larger than VOV then over-charging the battery may occur, which may present safety issues with LiIon batteries.
The conventional circuit 10 to implement precharging is shown in FIG. 1B. A precharge MOSFET 12 in series with a resistor 14 are used for precharging. At the time of precharging, charging FET 16 turns off and precharging FET 12 turns on. Therefore, the precharging current is approximately determined by the voltage difference between charger input voltage VPACK+ and total cell voltage Vcell, VPACK+−Vcell, divided by the serial resistor 14 Rpre. When the AC adapter is present and VPACK+ is higher than the cell voltage Vcell, the charging or precharging will start based on the initial voltage of each cell. If the voltage in any cell is lower than the threshold VUV, the battery pack will be in the precharging mode. Otherwise normal charging will be taken.
Those skilled in the art will recognize that the circuit 10 of FIG. 1B includes a battery monitor IC 20 that includes circuitry to monitor voltage and current conditions on each of the cells (Cell1, Cell2 . . . Cell4) of the battery pack 22. Such circuitry may include a switching network 24 to sample each cell voltage. To control the operation of the precharge MOSFET 12, the conventional circuit 10 includes a comparator 26 that compares a constant reference voltage 28 (VUV) with the voltage across each cell, via switches 30.
However, one drawback of the topology depicted in FIG. 1B is that an extra power MOSFET (i.e., MOSFET 12) and resistor 14 are required, which are expensive and increase PCB area. Additionally, with this topology, the lower the cell voltage results in a larger precharging current. Also, precharging current decreases with the increasing of cell voltage, which translates into longer time to finish precharging.
Additional, since the value of the resistor 14 is fixed, the maximum and minimum precharge current is also fixed, and cannot be adjusted to accommodate different battery pack requirements.